Since the error says that the file contains unknown nodes, it means at some point of time after this file was created,
atleast one node has been added to this file which was most probably by an external plugin. Sometimes, even if you delete objects
which are visible in the viewport, you might unknowingly leave behind some associated nodes. These nodes behave well as long
as the plugin in question is registered and available, but as soon as you delete the plugin, the same nodes become unknown nodes.
There are 2 ways to save a erroring binary .mb file as an ascii .ma file :
Solution 1 - By Preserving the unknown nodes
Open a new tab (python) in the script editor and type the following command :
Now try to save your file as an ascii, and you will succeed.
Solution 2 - By Deleting the unknown nodes :
You can delete unknown nodes in 2 ways, 1) by using Optimize Scene Size Options and 2) Using a simple script.
1) Using Optimize Scene Size Options :
Click the Options button in File/Optimize Scene Size. Scroll down to the bottom of the window that opens up.
Tick the check box Remove Unknown nodes, and then click the small "Optimize Now" button next to it.
This works most of the time, but sometimes you may find that some of the unknown nodes are locked, in which case
optimizer fails to delete it. If you are still unable to save your file as ascii, then use the script method that
I've written in the solution (2) below
2) A python script to delete unknown nodes ( including locked nodes) :
import maya.cmds as cmds
unknownNodes = cmds.ls(type="unknown")
for nd in unknownNodes:
print "Unlocking ", nd
cmds.lockNode( nd, lock=False )
print "Deleting ", nd
Hope that helps ....